Breastfeeding may have a positive effect on glucose metabolism and insulin sensitivity, which may reduce the risk of developing diabetes following gestational diabetes mellitus (GDM). This study aimed to evaluate the effect of breastfeeding and GDM on the body mass composition of the studied women, the levels of leptin, ghrelin, adiponectin, resistin, and insulin, and weight loss during the 6–8-week postpartum period and 1 year after childbirth. Materials and methods: The study group included 42 women with a singleton pregnancy, diagnosed with GDM between the 24th and 28th week of gestation. The control group consisted of 28 non-diabetic women with a singleton pregnancy. This study was carried out at 6–8 weeks as well as at 1 year postpartum. The women were subjected to body weight measurements and body composition analysis performed using a professional body composition analyzer TANITA DC-430 S MA. Waist circumference and subcutaneous fat was measured. Blood for laboratory tests was taken in the morning, on an empty stomach. Results: It was shown that, regardless of diabetes, exclusive breastfeeding had a significant impact on weight loss at 6–8 weeks postpartum (p = 0.014785) and lower insulin levels (p = 0.047). However, there was no effect of breastfeeding on the women’s anthropometric measurements or hormone levels one year after delivery, except for the thickness of subcutaneous adipose tissue, which was significantly lower in breastfeeding women (p = 0.03). One year after delivery, breastfeeding women had a lower BMI (p = 0.0014), less-thick subcutaneous adipose tissue (p < 0.001), and a lower risk of obesity (p = 0.016). There were also higher insulin and ghrelin levels in both breastfeeding and non-breastfeeding women (p < 0.001), and lower resistin levels in non-breastfeeding women (p = 0.004). Women who had diabetes during pregnancy had a significantly reduced waist circumference and subcutaneous fat thickness after one year (p < 0.001 and p = 0.05, respectively). Conclusions: Having diabetes during pregnancy did not significantly affect the results of anthropometric measurements and hormone levels noted at 6–8 weeks after delivery (the only exception was the thickness of subcutaneous fat tissue, which was greater in women without GDM). This may indicate normalization of carbohydrate metabolism after childbirth; however, the observation period is too short to elucidate long-term metabolic effects. This suggests the need for further research related to GDM and breastfeeding.